An increasing problem facing the semiconductor industry is that devices which rely on bulk properties of semiconductors will fail to retain their characteristic properties as feature sizes reach nanoscale dimension. The field of molecular electronics has grown significantly in the past few years in response to this reality. There has been much interest in developing molecule-based electronic materials for use in memory devices and as circuit elements. One approach has focused on memory applications wherein molecules attached to an electroactive surface serve as the active medium. The ability to store charge in the discrete redox states of molecules in effect mimics the function of semiconductor capacitors, such as those found in dynamic random access memory (DRAM) cells (which is comprised of a transistor-capacitor pair). This hybrid approach proceeds through transition-technologies wherein molecular materials are first integrated with semiconductors, thereby capitalizing on the vast infrastructure of the semiconductor industry.
Among several candidate redox-active molecules, we have found that porphyrinic macrocycles (e.g., porphyrins) are effective as the active memory element in molecular-based capacitors/storage cells. Porphyrins generally form robust, well-defined self-assembled monolayers (SAMs) on a variety of electroactive surfaces, display two (or three) stable cationic redox states of modest potential (0.5 V-1.2 V vs Ag/Ag+), readily participate in facile electron-transfer reactions, and display thermal stability, which is important as many chip processing steps require high temperatures. Several parameters are available for alteration. Specifically, we have examined the nature of the central metal, spacer group, surface-attachment group, and the non-linking meso-substituents. Alterations to one or more of these parameters can affect the redox-potentials, the number of redox-states, surface coverages, charge-injection rates, and charge-retention times of the resulting porphyrin SAMs.
Accordingly, due to the interest in molecular memory systems, improved methods of attachment and the compositions for such attachments are desirable.